Laser imaging systems are commonly used to produce photographic images from digital image data generated by magnetic resonance (MR), computed tomography (CT) or other types of medical image scanners. Systems of this type typically include a continuous tone laser imager for exposing the image on photosensitive film, a film processor for developing the film, and control subsystems for coordinating the operation of the laser imager and the film processor.
The digital image data is a sequence of digital image values representative of the scanned image. Image processing electronics within the control subsystem processes the image data values to generate a sequence of digital laser drive values (i.e., exposure values), which are input to a laser scanner. The laser scanner is responsive to the digital laser drive values for scanning across the photosensitive film in a raster pattern for exposing the image on the film.
The continuous-tone images used in the medical imaging field have very stringent image-quality requirements. A laser imager printing onto transparency film exposes an image in a raster format, the line spacing of which must be controlled to better than one micrometer. In addition, the image must be uniformly exposed such that the observer cannot notice any artifacts. In the case of medical imaging, the observers are professional image analysts (e.g., radiologists).
Film exposure systems are used to provide exposure of the image on photosensitive film. Known film exposure systems include a linear translation system and a laser or optical scanning system. The laser scanning system includes a laser scanner with unique optical configurations (i.e., lenses and mirrors) for exposure of the image onto the film. The linear translation system provides for movement of the laser scanning system in a direction perpendicular to the scanning direction, such that a full image may be scanned on a piece of photosensitive film.
In an internal drum type laser scanner assembly, a piece of film is positioned onto a film platen, wherein the film platen has a partial cylindrical or partial drum shape. The photosensitive film is positioned against the film platen. The laser or optical scanning system is positioned at the center of curvature of the photosensitive film for scanning a scan line across the photosensitive film surface. A linear translation system moves the laser or optical scanning system lengthwise along a longitudinal axis as defined by the center of curvature of the film to expose an entire image onto the film.
The film may be fed onto the film platen utilizing a film transport system which often incorporates a plurality of feed rollers. Once the piece of photosensitive film is fed onto the film platen, the film must be held tight against the curved surface of the film platen, and centered and aligned into a scanning position in order for an image to be correctly exposed onto the photosensitive film. Any skew of the film must also be removed. Often such methods and mechanisms for aligning and centering a piece of film on the internal surface of the film platen require multiple complex mechanical and electrical components and control systems.
U.S. Pat. No. 5,956,071, issued Sep. 21, 1999, inventors Mattila et al., discloses an assembly for positioning a film into a scanning position on a curved film platen in an internal drum scanner assembly. The film platen is defined by a first curved edge, a second curved edge, a film feed edge, and a film stop edge. The assembly comprises a first slider block assembly and a second slider block assembly which is spaced from the first slider block assembly a distance less than the width of the leading edge of the photosensitive film. A feed mechanism is positioned proximate the film feed edge, for feeding a piece of photosensitive film having a leading edge along the curved film platen. The leading edge of the film is fed from a location proximate the film feed edge towards the film stop edge.
When the photosensitive film is in the scanning position, the leading edge of the photosensitive film contacts the first slider assembly and the second slider assembly. The photosensitive film is tensioned against the curved film platen in alignment between the first slider assembly and the second slider assembly and the feed mechanism, thus removing any skew.
The laser scanning system and linear translation system must be protected from external vibration sources during the imaging process in order to minimize image degradation in the scanned film. Variation of placement of scan lines must be controlled very tightly to avoid banding artifacts. Vibration sources can effectively produce these same artifacts by exciting natural frequencies of the systems within the imaging assembly. More abrupt or short term sources, such as shock, can cause more visible artifacts at a given location on the scanned film. Therefore, to effectively manage the performance of the imaging assembly, vibration sources must be controlled. Prior designs to achieve isolation for internal drum scanning equipment typically uses commercially available vibration and shock mount made from various rubber-like materials. These components do not have low enough natural frequencies and also show significantly different values of natural frequency for the in-plane motion versus the normal direction. Air systems for achieving lower natural frequencies are typically cost prohibitive and sometimes undesirable in certain environments as leakage will disable the system.
There is thus a need for a vibration isolation system for a laser imaging system which is cost effective, which can control natural frequencies of the system in multiple directions, which has high performance, and which is reliable and minimally complex in design.